Brushless electric motors are finding increasing usage, particularly in automotive applications. Such motors are used or have potential for use in applications such as alternators, electronic throttle controls, electric power steering, fuel pumps, heater and air conditioner blower motors, and engine cooling fans. In a typical brushless motor, a permanent magnet(s) with a plurality of poles of alternating polarity is affixed to a rotor. The rotor is positioned inside a closely-fitting housing which carries electromagnets for propelling the rotor shaft.
In modern brushless electric motor designs, it is preferred to use a ring magnet of suitable permanent magnet composition assembled on a metal rotor such as a steel rotor. The use of a permanent magnet ring simplifies assembly of the structure and provides better structural integrity than mounting a plurality of individual permanent magnet arcs on the rotor. However, the design and assembly of a permanent magnet ring on a metal rotor to form a durable and efficient motor is not without its difficulties.
Permanent magnet rings are now typically formed of rare earth element-transition metal compositions such as neodymium-iron-boron and the like. Materials of these compositions can be formed into rings by sintering magnetically-aligned particles or by hot pressing or hot pressing and hot deforming the magnet particles into ring structures. Regardless of how the ring structures are formed, the designer must be aware that they are not as strong as rotor alloys and design the rotor and permanent magnet ring so that the ring can withstand the tensile stresses introduced into the ring by the high speed rotation of motor operation. However, there is a further and heretofore more difficult problem in the use of such permanent magnet rings on metal rotors. Normally, the coefficient of thermal expansion of the metal rotor is much greater than the coefficient of thermal expansion of the weaker magnet ring. It is desired to maintain as small a gap as possible between a permanent magnet ring and its rotor for purposes of maintaining a strong magnetic field. It is also desirable to maintain the smallest practical air gap between the outside diameter of the permanent magnet ring and the motor housing for the same reason. However, if care is not taken in the design and assembly of the permanent magnet ring on a steel rotor, any thermal expansion of the rotor is likely to break the permanent magnet ring. Heating of the rotor is very likely due to electrical heat generated in the operation of the motor and to environmental heating.
The permanent magnet rings of the iron-neodymium-boron type are usually processed in the form of magnetically-aligned anisotropic structures. Not only do these materials have lower coefficients of thermal expansion in the magnetically-aligned direction than steel, but in the direction transverse to magnetic alignment their coefficient of expansion may even be negative. It is these properties of the otherwise magnetically desirable iron-neodymium-boron type permanent magnets that complicate their use as ring magnets in brushless motors. Obviously, there is a need for improved practice in the assembly of permanent magnet rings on rotors for electric motors.
Heretofore, the solutions to assembling and retaining a permanent magnet ring on a metal rotor have taken two forms. In one solution, a strong metal alloy of low coefficient of thermal expansion, almost equal to that of the magnet material, is interposed as a ring between the steel rotor and the permanent magnet ring. This solution is illustrated, for example, in U.S. Pat. No. 5,402,025 to Saito et al. In the Saito et al construction, a rotor body is made of carbon steel. A strong but low thermal expansion ring made of 36 weight percent nickel, remainder iron (Invar) is bonded with an acrylic adhesive to the cylindrical surface of the steel rotor. Finally, a ring-shaped rare earth metal-iron permanent magnet is bonded to the nickel-iron ring by the use of the same acrylic-type adhesive. The difficulty with this kind of rotor permanent magnet structure is the increased cost of the Invar material and the cost of its assembly with the rotor and magnet ring. A second disadvantage is the loss in magnetic field properties due to the interposition of the nickel-iron layer between the permanent magnet and the high permeability iron rotor body.
A second solution to the problem of the steel rotor permanent magnet ring structure and usage is the provision of an outer ring of strong, relatively low thermal expansion material over the permanent magnet material, which in this instance is bonded directly to the steel rotor. In this case, the strong outer material applies a compressive stress during expansion of the permanent magnet ring which, hopefully, will prevent its structural failure. It also protects the machine from magnet chips in case of the magnet's disintegration inside the outer ring. Again, this solution requires the making and assembly of an extraneous component in the permanent magnet rotor assembly. The outer ring also requires a further spacing between the permanent magnet material and its opposing electromagnets on the housing of the motor body.